Abstract

<p><strong>Abstract.</strong> An improved algorithm for the retrieval of total and tropospheric nitrogen dioxide (NO<sub>2</sub>) columns from the Global Ozone Monitoring Experiment-2 (GOME-2) is presented. The refined retrieval will be implemented in a future version of the GOME Data Processor (GDP) as used by the EUMETSAT Satellite Application Facility on Atmospheric Composition and UV Radiation (AC-SAF). The first main improvement is the application of an extended 425&amp;ndash;497<span class="thinspace"></span>nm wavelength fitting window in the differential optical absorption spectroscopy (DOAS) retrieval of the NO<sub>2</sub> slant column density. Updated absorption cross-sections and a linear offset correction are used for the large fitting window. An improved slit function treatment is applied to compensate for both long-term and in-orbit drift of the GOME-2 slit function. Compared to the current operational (GDP 4.8) dataset, the use of these new features increases the NO<sub>2</sub> columns by <span class="thinspace"></span>1&amp;ndash;3<span class="thinspace"></span>&amp;times;<span class="thinspace"></span>10<sup>14</sup><span class="thinspace"></span>molec/cm<sup>2</sup> and reduces the slant column error by ∼<span class="thinspace"></span>24<span class="thinspace"></span>%. In addition, the bias between GOME-2A and GOME-2B measurements is largely reduced by adopting a new level 1b data version in the DOAS retrieval. The retrieved NO<sub>2</sub> slant columns show good consistency with the Quality Assurance for Essential Climate Variables (QA4ECV) retrieval with a good overall quality. Second, the STRatospheric Estimation Algorithm from Mainz (STREAM), which was originally developed for the TROPOspheric Monitoring Instrument (TROPOMI) instrument, was optimized for GOME-2 measurements to determine the stratospheric NO<sub>2</sub> column density. Applied to synthetic GOME-2 data, the estimated stratospheric NO<sub>2</sub> columns from STREAM shows a good agreement with the a priori truth. An improved latitudinal correction is introduced in STREAM to reduce the biases over the subtropics. Applied to GOME-2 measurements, STREAM largely reduces the overestimation of stratospheric NO<sub>2</sub> columns over polluted regions in the GDP 4.8 dataset. Third, the calculation of AMF applies an updated box-air mass factor (box-AMF) look-up table (LUT) calculated using the latest version of VLIDORT model with an increased number of reference points and vertical layers, a new GOME-2 surface albedo climatology, improved a priori NO<sub>2</sub> profiles obtained from the TM5-MP chemistry transport model, and improved GOME-2 cloud parameters. A large effect on the retrieved tropospheric NO<sub>2</sub> columns (more than 10<span class="thinspace"></span>%) is found over polluted regions. To evaluate the GOME-2 tropospheric NO<sub>2</sub> columns, an end-to-end validation is performed using ground-based multiple-axis DOAS (MAXDOAS) measurements. The validation is illustrated for 6 stations covering urban, suburban, and background situations. Compared to the GDP 4.8 product, the new dataset presents an improved agreement with the MAXDOAS measurements for all the stations.</p>